A retinal prosthesis is a device implanted in the eyes of blind patients to stimulate retinal neurons that have survived degeneration, causing those neurons to transmit artificial visual signals to the brain. If the artificial visual signals closely mimic natural retinal signals, the device could provide useful artificial vision to millions of blind patients. A major limitation of prototype prostheses now in clinical trials is that they stimulate many neurons of different types indiscriminately and simultaneously. This is problematic because in the normal retina, roughly 20 different types of retinal ganglion cells send different types of visual information to diverse targets in the brain, and these cell types are intermingled in the retina. Four major ganglion cell types (ON-midget, OFF-midget, ON-parasol, OFF-parasol) form about 70% of the visual signals conveyed to the brain and supply neural signals to brain areas responsible for high resolution vision. In normal vision, these cells produce different signals at different times. For example, ON and OFF cells respond to increments and decrements of light, respectively; midget cells exhibit slower and less transient responses than parasol cells. Thus, current prostheses that activate different cell types simultaneously and indiscriminately produce abnormal retinal signals. Such a device is of limited utility to the patient.